Among metal-semiconductor contacts, there are two main groups: ohmic contacts and Schottky contacts. An ohmic contact is referred to as a metal-semiconductor contact that has negligible contact resistance relative to the series resistance of the semiconductor and a small voltage drop over the metal-semiconductor contact as compared to the total voltage drop across a device comprising the metal-semiconductor contact. A Schottky contact comprises a rectifying metal-semiconductor junction, referred to as a Schottky barrier or a Schottky diode. The barrier height is determined by the difference between the work function of the metal part and the electron affinity of the semiconductor part of the junction.
The Schottky diode has many advantages over conventional pn diodes, one of them being that it is a majority carrier device, in contrast to pn diodes where current transport is due to minority carriers. Thus the device exhibits no minority carrier storage effects, making it a very attractive choice in high speed applications. In addition, since the Schottky diode is a majority carrier device and the current flow mechanism is that of thermionic emission over a potential barrier, the turn-on voltage is defined almost entirely by the metal work function, the electron affinity of the semiconductor and surface states at the junction. This gives lower low turn-on voltage and higher reverse-saturation current density than in a pn diode.
Wide-bandgap semiconductors are particularly suitable for Schottky diodes. When compared to Si they offer improved performance in terms of breakdown voltage, lower leakage currents, higher temperature stability, faster reverse recovery times and positive temperature coefficients of resistance. The latter is useful for preventing thermal runaway in parallel diode applications. Taking all those advantages in consideration, it is understood that widespread adoption of wide-bandgap Schottky diodes would mean significant improvement in efficiency and lower power consumption in applications such as for example switch-mode power supplies. Another advantage is that the total package size can be made smaller due to the possibility to reduce the size of heat sinks, since wide-bandgap Schottky diodes can operate at higher temperatures as compared to the Si counterparts.
Among the wide-bandgap semiconductors available, there is a particular interest for SiC and GaN in Schottky diode applications. High performance Schottky diodes have been described for example in CA2515173 and EP1947700, where the diodes are manufactured from epitaxial GaN grown on a GaN substrate and from epitaxial AlGaN grown on a SiC substrate. In addition, U.S. Pat. No. 6,768,146 describes Schottky diodes made from GaN grown on sapphire substrates. However, problem arises when using the material combinations described. The most significant issue is the cost. These devices become very expensive to manufacture due to the high cost of the substrates. In addition to that, SiC requires very high growth temperatures, over 1500° C., which also significantly increases production cost.
Recently Schottky diodes comprising nanowires as part of the metal-semiconductor junction has been demonstrated. WO 2005/124872 discloses a Schottky diode formed by semiconductor nanowires of single conductivity type grown on a substrate and a metal contact arranged on the opposed end of the semiconductor nanowires. WO 2004/109815 discloses Schottky diodes comprising arrays of semiconductor nanowires with metal deposited on the tips thereof in order to form a metal-semiconductor junction. In WO 2007/021069 a metal contact layer is formed on top of an array of semiconductor nanowires with pn junctions in order to form a Schottky contact. These nanowire Schottky diodes benefit from the small cross sectional area of the nanowires to accomplish a small device area and inevitably also from using wide-bandgap semiconductors such as GaN. Moreover, it is known from WO 2007/021069 that problems caused by lattice mismatch due to GaN growth can be avoided by using nanowires.